Actually I do think it fair. The same way that as a former chemist I'd be critical of a school that left acids where kids could grab them.
You don't make a system more usable by making it insecure. Accidental things by kids or adults will cause hassles. I've mostly worked with responsible office workers, but they wreak occasional havoc accidentally.
I accept that the person doing this was following orders, and to be sure there is blame to be shared, but as someone who frequently has come under that sort of pressure from senior execs, my job is to stop them from screwing things up, just in a more oblique way.
Why are teachers files in public areas ? That's not "usability" that's incompetence.
Let's not get too techie here, let's just see these files as pieces of paper.
If a teacher left sensitive reports about kids in the middle of the playground, she'd would rightly be thought at least a bit daft.
But we all have left things down in the wrong place.
If she then discovered the kids reading them, but did nothing except occasionally shouting at them, then I think we'd really begin to question her judgement.
If weeks later, she was still "storing" her files in a way that not only allowed kids access to sensitive information, but also allowed them to change it, then I don't think anyone would see her as competent.
It is the work of a few minutes to simply create a type of user called "Teacher". All teachers can access everything. Not ideal, but easy.
You then create a group of users called kids. They can't access the control panel at all, and only files in their own private area.
You can also easily block them from installing any software.
That will keep out any 6 yo, and all but the most gifted 11 yos, without getting in the way of staff.